ORDER, DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM Magnetocaloric Effect in Manganites

The magnetocaloric effect (MCE) in La 1- xSrxMnO3, Sm0.55Sr0.45MnO3, and PrBaMn2O6 com� pounds is studied. The maximum values of MCE (ΔTmax) determined by a direct method in the second and third compositions and in La0.9Sr0.1MnO3 are found to be much lower than those calculated from the change of the magnetic part of entropy in the Curie temperature (TC) and the Neel temperature (TN) range. The neg� ative contribution of the antiferromagnetic (AFM) part of a sample in the La 1- x Sr x MnO 3 system at 0.1 ≤ x ≤ 0.3 decreases ΔTmax and changes the ΔT(T) curve shape, shifting its maximum 20-40 K above TC. Lower val� ues of ΔT max are detected in the range T C = 130-142 K in polycrystalline and singlecrystal Sm 0.55 Sr 0.45 MnO 3 samples cooled in air. If such samples were cooled in an oxygen atmosphere (which restores broken Mn-O- Mn bonds and, thus, increases the volume of CEtype AFM clusters), the maximum in the temperature dependence of MCE is located at T N (243 K) for CEtype AFM clusters. A magnetic field applied to a sample during the MCE measurements transforms these clusters into a ferromagnetic (FM) state, and both types of clusters decompose at T = TN. The PrBaMn2O6 composition undergoes an AFM-FM transition at 231 K, and the temperature dependence of its MCE has a sharp minimum at T = 234 K, where MCE is negative, and a broad maximum covering TC. The absolute values of MCE at both extrema are several times lower than those calculated from the change in the magnetic entropy. These phenomena are explained by the presence of a magnetically heterogeneous FM-AFM state in these manganites.